Synthesis, Characterization, and Reduction of Thorium Pyridinediimine Complexes.

A family of thorium complexes featuring the redox-noninnocent pyridinediimine ligand MesPDIMe was synthesized, including (MesPDIMe)ThCl4 (1-Th), (MesPDIMe)ThCl3(THF) (2-Th), (MesPDIMe)ThCl2(THF)2 (3-Th) and [(MesPDIMe)Th(THF)]2 (5-Th) Full characterization of these species shows that these complexes feature MesPDIMe in four different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, X-ray crystallography, and SQUID magnetometry where appropriate.

A Bidentate Ligand Featuring Ditopic Lewis Acids in the Second Sphere for Selective Substrate Capture and Activation.

We present a ligand platform featuring appended ditopic Lewis acids to facilitate capture/activation of diatomic substrates. We show that incorporation of two 9-borabicyclo[3.3.1]nonane (9-BBN) units on a single carbon tethered to a pyridine pyrazole scaffold maintains a set of unquenched nitrogen donors available to coordinate FeII , ZnII , and NiII . Using hydride ion affinity and competition experiments, we establish an additive effect for ditopic secondary sphere boranes, compared to the monotopic analogue. These effects are exploited to achieve high selectivity for binding NO2 - in the presence of competitive anions such as F- and NO3 - . Finally, we demonstrate hydrazine capture within the second-sphere of metal complexes, followed by unique activation pathways to generate hydrazido and diazene ligands on Zn and Fe, respectively.

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid.

We disclose a 1,4,7-triazacyclononane (TACN) ligand featuring an appended boron Lewis acid. Metalation with Cu(I) affords a series of tetrahedral complexes including a boron-capped cuprous hydride. We demonstrate distinct reactivity modes as a function of chemical oxidation: hydride transfer to CO2 in the copper(I) state and oxidant-induced H2 evolution as well as alkyne reduction.

Hydrazone and Oxime Olefination via Ruthenium Alkylidenes.

We describe the development of an efficient method for the olefination of hydrazones and oximes. The key design approach that enables this transformation is tuning of the energy/polarity of C=N π-bonds by employing heteroatom functionalities (NR2 , OR). The resulting hydrazones or oximes facilitate olefination with ruthenium alkylidenes. Through this approach, we show that air-stable, commercially available ruthenium alkylidenes provide access to functionalized alkenes (20 examples) in ring-closing reactions with yields up to 88 %.

Substrate Specific Metal-Ligand Cooperative Binding: Considerations for Weak Intramolecular Lewis Acid/Base Pairs.

Metal-ligand cooperative binding modes were interrogated in a series of zinc bis(thiophenoxide) complexes. A weak B-S binding interaction is observed in solution between the weakly Lewis basic thiophenoxide ligands and an appended trialkylborane. The energy of this binding event is dependent upon the strength of the Lewis acid and its proximity to the zinc thiophenoxide.

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization.

We outline the generality and requirements for cooperative N2H4 capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH2 stabilization. While the Lewis acids are required to bind N2H4, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

Requirements for Lewis Acid-Mediated Capture and N-N Bond Cleavage of Hydrazine at Iron.

An iron complex bearing a pyridine(dicarbene) pincer was designed to probe the requirements of Lewis acid-enabled N2H4 capture and subsequent N-N bond cleavage. Appended boron Lewis acids were installed by two methods to circumvent the incompatibilities associated with Lewis acid/base quenching of free carbenes and boranes. N2H4 capture by borane Lewis acids is dependent on both the Lewis acidity and the steric profile about boron. A substitutionally inert primary coordination sphere at iron prevents an Fe-N2H4 interaction as well as N-N bond homolysis upon reduction.

Hydrogen Bonds Dictate O2 Capture and Release within a Zinc Tripod.

Six directed hydrogen bonding (H-bonding) interactions allow for the reversible capture and reduction of dioxygen to a trans-1,2-peroxo within a tripodal zinc(II) framework. Spectroscopic studies of the dizinc peroxides, as well as on model zinc diazides, suggest H-bonding contributions serve a dominant role for the binding/activation of these small molecules.

Tailoring the Electronic Structure of Uranium Mono(imido) Species through Ligand Variation.

Uranium mono(imido) species have been prepared via the oxidation of Cp*U(MesPDIMe)(THF) (1-Cp*) and [CpPU(MesPDIMe)]2 (1-CpP), where Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide, CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide, MesPDIMe = 2,6-[(Mes)N═CMe]2C5H3N, and Mes = 2,4,6-trimethylphenyl, with organoazides. Treating either with N3DIPP (DIPP = 2,6-diisopropylphenyl) formed uranium(IV) mono(imido) complexes, CpPU(NDIPP)(MesPDIMe) (2-CpP) and Cp*U(NDIPP)(MesPDIMe) (2-Cp*), featuring reduced [MesPDIMe]-. The addition of electron-donating 1-azidoadamantane (N3Ad) to 1-Cp* generated a dimeric product, [Cp*U(NAd)(MesHPDIMe)]2 (3), from radical coupling at the p-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated. To support this, Cp*U(tBu-MesPDIMe)(THF) (1-tBu), which has a tert-butyl group protecting the para position, was also treated with N3Ad, and the monomeric product, Cp*U(NAd)(tBu-MesPDIMe) (2-tBu), was isolated. All isolated complexes were analyzed spectroscopically and structurally, and the dynamic solution behavior was examined using electronic absorption spectroscopy.

Hydrazine Capture and N-N Bond Cleavage at Iron Enabled by Flexible Appended Lewis Acids.

Incorporation of two 9-borabicyclo[3.3.1]nonyl substituents within the secondary coordination sphere of a pincer-based Fe(II) complex provides Lewis acidic sites capable of binding 1 or 2 equiv of N2H4. Reduction of the 1:1 Fe:N2H4 species affords a rare Fe(NH2)2 complex in which the amido ligands are stabilized through interactions with the appended boranes. The NH2 units can be released as NH3 upon protonation and exchanged with exogenous N2H4.

Facile Reductive Silylation of UO22+ to Uranium(IV) Chloride.

General reductive silylation of the UO22+ cation occurs readily in a one-pot, two-step stoichiometric reaction at room temperature to form uranium(IV) siloxides. Addition of two equivalents of an alkylating reagent to UO2 X2 (L)2 (X=Cl, Br, I, OTf; L=triphenylphosphine oxide, 2,2'-bipyridyl) followed by two equivalents of a silyl (pseudo)halide, R3 Si-X (R=aryl, alkyl, H; X=Cl, Br, I, OTf, SPh), cleanly affords (R3 SiO)2 UX2 (L)2 in high yields. Support is included for the key step in the process, reduction of UVI to UV . This procedure is applicable to a wide range of commercially available uranyl salts, silyl halides, and alkylating reagents. Under this protocol, one equivalent of SiCl4 or two equivalents of Me2 SiCl2 results in direct conversion of the uranyl to uranium(IV) tetrachloride. Full spectroscopic and structural characterization of the siloxide products is reported.

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species.

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [CpPU(MesPDIMe)]2 (1-CpP), Cp*U(MesPDIMe)(THF) (1-Cp*) (CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(tBu-MesPDIMe) (THF) (1-tBu) (2,6-((Mes)N═CMe)2-p-R-C5H2N, Mes = 2,4,6-trimethylphenyl; R = H, MesPDIMe; R = C(CH3)3, tBu-MesPDIMe), has been investigated. While 1-Cp* and 1-CpP readily reduce N3R (R = Ph, p-tolyl) to form trans-bis(imido) species, CpPU(NAr)2(MesPDIMe) (Ar = Ph, 2-CpP; Ar = p-Tol, 3-CpP) and Cp*U(NPh)2(MesPDIMe) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-CpP, and 3-CpP are uranium(V) trans-bis(imido) species supported by neutral [MesPDIMe]0 ligands formed by complete oxidation of [MesPDIMe]3- ligands of 1-CpP and 1-Cp*. Variation of the arylimido substituent in 2-Cp* from phenyl to p-tolyl, forming Cp*U(NTol)2(MesPDIMe) (3-Cp*), changes the electronic structure, generating a uranium(VI) ion with a monoanionic pyridine(diimine) radical. The tert-butyl-substituted analogue, Cp*U(NTol)2(tBu-MesPDIMe) (3-tBu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3-tBu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol)2(MesPDIMe)][SbF6] (4-Cp*) and [Cp*U(NTol)2(tBu-MesPDIMe)][SbF6] (4-tBu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF6 affords cationic [Cp*U(NPh)2(MesPDIMe)][SbF6] (5-Cp*) from a metal-based U(V)/U(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of 1-CpP, 3-Cp*, 3-tBu, 4-Cp*, 4-tBu, and 5-Cp* have been elucidated by X-ray crystallography.

Elucidating the Mechanism of Uranium Mediated Diazene N═N Bond Cleavage.

Investigation into the reactivity of reduced uranium species toward diazenes has revealed key intermediates in the four-electron cleavage of azobenzene. Trivalent Tp*2U(CH2Ph) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*2U(2,2'-bpy) (1b) both perform the two-electron reduction of diazenes affording η2-hydrazido complexes Tp*2U(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*2U(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U(MesPDIMe)(THF) (3) (MesPDIMe = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly reduced trianionic chelate that undergoes electron transfer. This proceeds via concerted addition at a single uranium center supported by both a crossover experiment and through addition of an asymmetrically substituted diazene, Ph-N═N-Tol. Further investigation of 3 and its substituted analogue, Cp*U(tBu-MesPDIMe)(THF) (3-tBu) (tBu-MesPDIMe = 2,6-((Mes)N═CMe)2-p-C(CH3)3-C5H2N), with benzo[c]cinnoline, revealed that the four-electron cleavage occurs first by a single electron reduction of the diazene with the redox chemistry performed solely at the redox-active pyridine(diimine) to form dimeric [Cp*U(BCC)(MesHPDIMe)]2 (5) and Cp*U(BCC)(tBu-MesPDIMe) (6). While a transient pyridine(diimine) triplet diradical in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety in 6 allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U(BCC)(MesPDIMe) (7), was synthesized via salt metathesis from Cp*UI(MesPDIMe) (3-I). All complexes have been characterized by 1H NMR and electronic absorption spectroscopies, X-ray diffraction, and, where pertinent, EPR spectroscopy. Further, the electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.

Synthesis, Characterization, and Stoichiometric U-O Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand Radicals.

Two uranium(VI) uranyl compounds, Cp*UO2((Mes)PDI(Me)) (3) and Cp*UO2((t)Bu-(Mes)PDI(Me)) (3-(t)Bu) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; (Mes)PDI(Me) = 2,6-((Mes)N=CMe)2C5H3N; (t)Bu-(Mes)PDI(Me) = 2,6-((Mes)N=CMe)2-p-C(CH3)3C5H2N; Mes = 2,4,6-trimethylphenyl), have been synthesized by addition of N-methylmorpholine N-oxide to trianionic pyridine(diimine) uranium(IV) precursors, Cp*U((Mes)PDI(Me))(THF) (1), Cp*U((Mes)PDI(Me))(HMPA) (1-HMPA), and Cp*U((t)Bu-(Mes)PDI(Me))(THF) (1-(t)Bu). These uranyl complexes contain singly reduced pyridine(diimine) ligands suggesting formation occurs via cooperative ligand/metal oxidation. Treating 3 or 3-(t)Bu with stoichiometric equivalents of Me3SiI results in stepwise oxo silylation to form (Me3SiO)2UI2((Mes)PDI(Me)) (5) or (Me3SiO)UI2((t)Bu-(Mes)PDI(Me)) (5-(t)Bu), respectively. Additional equivalents result in full uranium-oxo bond scission and formation of UI4(1,4-dioxane)2 with extrusion of hexamethyldisiloxane. The uranium complexes have been characterized via multinuclear NMR, vibrational, and electronic absorption spectroscopies and, in some cases, X-ray crystallography.

Investigation of Uranium Tris(imido) Complexes: Synthesis, Characterization, and Reduction Chemistry of [U(NDIPP)3(thf)3].

Addition of KC8 to trivalent [UI3(thf)4] in the presence of three equivalents of 2,6-diisopropylphenylazide (N3DIPP) results in the formation of the hexavalent uranium tris(imido) complex [U(NDIPP)3(thf)3] (1) through a facile, single-step synthesis. The X-ray crystal structure shows an octahedral complex that adopts a facial orientation of the imido substituents. This structural trend is maintained during the single-electron reduction of 1 to form dimeric [U(NDIPP)3{K(Et2O)}]2 (2). Variable-temperature/field magnetization studies of 2 show two independent U(V) 5f (1) centers, with no antiferromagnetic coupling present. Characterization of these complexes was accomplished using single-crystal X-ray diffraction, variable-temperature (1)H NMR spectroscopy, as well as IR and UV/Vis absorption spectroscopic studies.

Trivalent uranium phenylchalcogenide complexes: exploring the bonding and reactivity with CS2 in the Tp*2UEPh series (E = O, S, Se, Te).

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U-E bond. All compounds have been characterized by (1)H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry. Computational studies establish Mulliken spin densities for the uranium centers ranging from 3.005 to 3.027 (B3LYP), consistent for uranium-chalcogenide bonds that are primarily ionic in nature, with a small covalent contribution. The reactivity of 2-4 toward carbon disulfide was also investigated and showed reversible CS2 insertion into the U(III)-E bond, forming Tp*2U(κ(2)-S2CEPh) (E = S (5), Se (6), Te (7)). Compound 5 was characterized crystallographically.

Multielectron C-O bond activation mediated by a family of reduced uranium complexes.

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp(P) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type Cp(X)UI2((Mes)PDI(Me)) (1-Cp(X); X = * or P), Cp(X)UI((Mes)PDI(Me)) (2-Cp(X)), and Cp(X)U((Mes)PDI(Me))(THF)n (3-Cp(X); *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of (Mes)PDI(Me) reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp(X), which has a doubly reduced (Mes)PDI(Me), with furfural results in radical coupling between the substrate and (Mes)PDI(Me), leading to C-C bond formation to form Cp(X)UI((Mes)PDI(Me)-CHOC4H3O) (4-Cp(X)). Exposure of 3-Cp* and 3-Cp(P), which contain a triply reduced (Mes)PDI(Me) ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)((Mes)PDI(Me)) (5-Cp*) and Cp(P)U(O2C2Ph4)((Mes)PDI(Me)) (5-Cp(P)). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1-5 have been characterized by (1)H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [(Mes)PDI(Me)](-) ligand in 1-Cp(P) and 5-Cp(P). Structural parameters of complexes 1-Cp(P), 2-Cp(X), 4-Cp*, and 5-Cp(X) have been elucidated by X-ray crystallography.

Synthesis of terminal uranium(IV) disulfido and diselenido compounds by activation of elemental sulfur and selenium.

Rare stakes: Terminal uranium(IV) disulfido and diselenido compounds, Tp*2U(E2) (E=S, Se), were synthesized by the activation of elemental chalcogens. Structural, spectroscopic, computational and magnetic studies of these species establish their tetravalency and highly polarized U-E bonds.

Mobility of Lewis acids within the secondary coordination sphere: toward a model for cooperative substrate binding.

Distance dependence of appended Lewis acids in N2H4 binding and deprotonation was evaluated within a series of zinc complexes. Variation of spacer-length to a tethered trialkylborane Lewis acid revealed distinct preferences for binding and stabilization of the resulting deprotonated N2H3- unit.

Tetrahedral iron featuring an appended Lewis acid: distinct pathways for the reduction of hydroxylamine and hydrazine.

We present the preparation of a nitrogen-based bidentate ligand featuring an appended boron Lewis acid as well as its tetrahedral Fe2+ and Zn2+ complexes. These complexes act as platforms for hydrazine and hydroxylamine capture and reduction chemistry.